High-carbon steel wire rod and wire excellent in drawability and methods of producing the same

ABSTRACT

This invention provides high-carbon steel wire rod and wire excellent in drawability and methods of producing the same. 
     The high-carbon steel wire rod or wire is characterized in that it contains, in weight percent, C: 0.90-1.10%, Si: not more than 0.40% and Mn: not more than 0.50%, is limited to P: not more than 0.02%, S: not more than 0.01% and Al: not more than 0.003%, the remainder being Fe and unavoidable impurities, and has a microstructure of, in terms of area ratio, not less than 80% upper bainite texture obtained by two-stepped transformation and an Hv of not more than 450. The high-carbon steel wire rod or wire may additionally contain Cr: 0.10-0.30% as an alloying component.

TECHNICAL FIELD

This invention relates to high-carbon steel wire rod and wire excellentin drawability and methods of producing the same.

BACKGROUND ART

Wire rod and wire are ordinarily drawn into a final product matched tothe purpose of use. Before conducting the drawing process, however, itis necessary to put the wire rod or wire in a condition for drawing.

As a conventional measure for this, Japanese Patent Publication No. Sho60-56215 discloses a method for heat treatment of steel wire rod of highstrength and small strength variance characterized in that wire rod ofsteel containing C: 0.2-1.0%, Si<0.30% and Mn: 0.30-0.90% and ataustenite formation temperature is cooled between 800° and 600° C. at acooling rate of 15°-60° C./sec by immersion in fused salt of one or bothof potassium nitrate and sodium nitrate fused by heating to atemperature of 350°-600° C. and stirred by a gas.

However, the wire rod of pearlite texture obtained by the heat treatmentmethod described in the aforesaid patent publication involves theproblems of ductility degradation during drawing at a high reduction ofarea and of cracking in twist testing (hereinafter referred to as"delamination").

The object of this invention is to provide high-carbon steel wire rodand wire excellent in drawability and methods of producing the samewhich advantageously overcome the aforesaid problems of the prior art.

DISCLOSURE OF THE INVENTION

The gist of the invention is as set out below.

(1) High-carbon steel wire rod or wire excellent in drawabilitycharacterized in that

it contains, in weight percent,

C: 0.90-1.10%,

Si: not more than 0.40% and

Mn: not more than 0.50%,

is limited to

P: not more than 0.02%,

S: not more than 0.01% and

Al: not more than 0.003%,

the remainder being Fe and unavoidable impurities, and has amicrostructure of, in terms of area ratio, not less than 80% upperbainite texture obtained by two-stepped transformation and an Hv of notmore than 450.

(2) High-carbon steel wire rod or wire excellent in drawabilityaccording to paragraph 1 above further containing Cr: 0.10-0.30% as analloying component.

(3) A method of producing high-carbon steel wire rod excellent indrawability characterized by,

rolling into wire rod a steel slab of a composition which

contains, in weight percent,

C: 0.90-1.10%,

Si: not more than 0.40% and

Mn: not more than 0.50%,

is limited to

P: not more than 0.02%,

S: not more than 0.01% and

Al: not more than 0.003%,

the remainder being Fe and unavoidable impurities,

cooling the rolled wire rod from the temperature range of 1100°-755° C.to the temperature range of 350°-500° C. at a cooling rate of 60°-300°C./sec, and

holding it in this temperature range for a specified time period withinthe range in which bainite transformation does not begin or within arange from after the start of bainite transformation to prior tocompletion of bainite transformation, and

increasing the temperature and holding it until bainite transformationis completely finished.

(4) A method of producing high-carbon steel wire rod excellent indrawability according to paragraph 3 above wherein the starting slabfurther contains Cr: 0.10-0.30% as an alloying component.

(5) A method of producing high-carbon steel wire rod excellent indrawability according to paragraph 3 or 4 above characterized by,

after the starting slab has been rolled into wire rod, cooling therolled wire rod from the temperature range of 1100°-755° C. to thetemperature range of 350°-500° C. at a cooling rate of 60°-300° C./sec,

holding it in this temperature range for not less than 1 sec and notmore than a period within the range in which bainite transformation doesnot begin of X sec determined by the following equation (1), and

increasing the temperature not less than 10° C. and not more than 600-T₁(T₁ : holding temperature after cooling) °C. and holding it untilbainite transformation is completely finished,

    X=exp (16.03-0.0307×T.sub.1)                         (1)

where

T₁ : holding temperature after cooling.

(6) A method of producing high-carbon steel wire rod excellent indrawability according to paragraph 3 or 4 above characterized by,

after the starting slab has been rolled into wire rod, cooling therolled wire rod from the temperature range of 1100°-755° C. to thetemperature range of 350°-500° C. at a cooling rate of 60°-300° C./sec,

holding it in this temperature range for a period from after the startof bainite transformation to prior to completion of bainitetransformation, specifically for a period of not more than Y secdetermined by the following equation (2), and

increasing the temperature not less than 10° C. and not more than 600-T₁(T₁ : holding temperature after cooling) °C. and holding it untilbainite transformation is completely finished,

    Y=exp (19.83-0.0329×T.sub.1)                         (2)

where

T₁ : holding temperature after cooling.

(7) A method of producing high-carbon steel wire excellent indrawability characterized by,

heating to the temperature range of 1100°-755° C. wire of a compositionwhich

contains, in weight percent,

C: 0.90-1.10%,

Si: not more than 0.40% and

Mn: not more than 0.50%,

is limited to

P: not more than 0.02%,

S: not more than 0.01% and

Al: not more than 0.003%,

the remainder being Fe and unavoidable impurities,

cooling the heated wire to the temperature range of 350°-500° C. at acooling rate of 60°-300° C./sec, and

holding it in this temperature range for a specified time period withinthe range in which bainite transformation does not begin or within arange from after the start of bainite transformation to prior tocompletion of bainite transformation, and

increasing the temperature and holding it until bainite transformationis completely finished.

(8) A method of producing high-carbon steel wire excellent indrawability according to paragraph 7 above wherein the starting wirefurther contains Cr: 0.10-0.30% as an alloying component.

(9) A method of producing high-carbon steel wire excellent indrawability according to paragraph 7 or 8 above characterized by,

cooling the starting wire from the temperature range of 1100°-755° C. tothe temperature range of 350°-500° C. at a cooling rate of 60°-300°C./sec,

holding it in this temperature range for not less than 1 sec and notmore than a period within the range in which bainite transformation doesnot begin of X sec determined by the following equation (1), and

increasing the temperature not less than 10° C. and not more than 600-T₁(T₁ : holding temperature after cooling) °C. and holding it untilbainite transformation is completely finished,

    X=exp (16.03-0.0307×T.sub.1)                         (1)

where

T₁ : holding temperature after cooling.

(10) A method of producing high-carbon steel wire excellent indrawability according to paragraph 7 or 8 above characterized by,

cooling the starting wire from the temperature range of 1100°-755° C. tothe temperature range of 350°-500° C. at a cooling rate of 60°-300°C./sec,

holding it in this temperature range for a period from after the startof bainite transformation to prior to completion of bainitetransformation, specifically for a period of not more than Y secdetermined by the following equation (2), and

increasing the temperature not less than 10° C. and not more than 600-T₁(T₁ : holding temperature after cooling) °C. and holding it untilbainite transformation is completely finished,

    Y=exp (19.83-0.0329×T.sub.1)                         (2)

where

T₁ : holding temperature after cooling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a heat treatment pattern of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained in detail in the following.

The reasons for the restrictions on the bainite high-carbon steel wirerod and wire for drawing and the method of producing the same accordingto this invention will now be discussed.

The inventors discovered that in ordinary patenting treatmentpro-eutectoid cementite is precipitated along the old austenite grainboundaries even in an eutectoid composition with a C content in thevicinity of 0.8% and that this pro-eutectoid cementite becomes a causeof ductility degradation after drawing. C is an economical and effectivestrengthening element and is also an effective element for lowering theamount of this pro-eutectoid cementite precipitated. Therefore, theamount of added C has to be set at not less than 0.90% for an ultra-finewire of a tensile strength of not less than 3500 MPa and enhancedductility. Since ductility is reduced and drawability degraded when theamount of addition is too high, however, the upper limit is set at1.10%.

Si is an element required for deoxidizing the steel and the deoxidizingeffect is therefore insufficient when the amount contained is too small.In addition, Si increases the strength after patenting by entering insolid solution into the ferrite phase in the pearlite formed after heattreatment. On the other hand, however, when the content is too large, itlowers the ductility of the ferrite and, in turn, lowers the ductilityof the ultra-fine wire after drawing. The upper limit of its content istherefore set at 0.40%.

A small amount of Mn is preferably added for securing hardenability.However, addition of a large amount of Mn induces segregation, leadingto formation of supercooled textures, namely bainite and martensite,which thereafter impair drawability. The content is therefore set at notmore than 0.50%.

For securing ductility on a par with prior art ultra-fine wire, Scontent is set at not more than 0.01%. Since, like S, P also impairs theductility of wire rod or wire, its content is set at not more than0.02%.

Presence of nonductile inclusions whose main component is Al₂ O₃, suchas Al₂ O₃, MgO--Al₂ O₃ and the like, is a cause for reduction ofultra-fine wire ductility. In this invention, therefore, Al content isset at not more than 0.003% for avoiding ductility reduction bynonductile inclusions.

In the case of a hypereutectoid steel such as that of this invention, acementite network easily forms in the texture following patenting andthick cementite precipitates readily occur. For achieving high strengthand high ductility in such a steel, it is necessary to make the pearlitefine and, further, to eliminate the aforesaid cementite network and thethick cementite. The Cr added as occasion demands in this inventionsuppresses the appearance of such abnormal cementite portions and hasthe further effect of making the pearlite fine. However, addition of alarge amount increases the dislocation density in the ferrite followingheat treatment and thus markedly degrades the ductility of theultra-fine wire following drawing. The Cr content is therefore set atnot less than 0.10%, at which its effect can be anticipated, and notmore than 0.30%, at which ductility is not degraded by increaseddislocation density in the ferrite.

The reasons for the limitations in the production method of the presentinvention are as follows.

The cooling start temperature (T₀) following wire rod rolling orfollowing wire heating affects the texture following transformation. Thelower limit is set at not less than the austenite transformation point(755° C.), which is the equilibrium transformation start temperature.The upper limit is set at 1100° C. for suppressing abnormal austenitegrain growth.

The cooling rate (V₁) following wire rod rolling or following wireheating is an important factor in suppressing the start of pearlitetransformation. This was experimentally ascertained by the inventors. Inthe case of gradual cooling at an initial cooling rate of less than 60°C./sec, transformation starts on the high-temperature side of thepearlite transformation nose position, making it impossible to obtain aperfect bainite texture owing to formation of pearlite texture. Whilebainite texture forms at temperature under 500° C., formation of aperfect bainite texture requires rapid cooling at the initial coolingstage. The lower limit of the cooling rate (V₁) is therefore set at 60°C./sec, while the upper limit thereof is set at the industriallyfeasible 300° C./sec.

The isothermal holding temperature (T₁) after cooling is an importantfactor determining the formed texture. At a holding temperatureexceeding 500° C., pearlite texture forming at the center portion of thewire rod or wire increases tensile strength and degrades drawability. Ata holding temperature below 350° C., granulation of cementite in thebainite structure starts, increasing tensile strength and degradingdrawability. The upper limit of the isothermal transformationtemperature is therefore set at 500° C. and the lower limit thereof isset at 350° C.

Supercooled austenite texture is obtained by holding at 350°-500° C. fora specified period of time. When the temperature is increasedthereafter, the cementite precipitation in the bainite texture whichappears is coarser than in isothermal transformation. As a result, thetwo-step-transformed upper bainite texture softens.

In the case of complete two-stepped transformation, the super coolingtime (t₁) required in the temperature range of 350°-500° C. is not lessthan the time required for formation of supercooled austenite and theupper limit thereof is up to prior to the start of bainitetransformation. It is preferably not less than 1 sec and not more than Xsec indicated by the following equation:

    X=exp (16.03-0.0307×T.sub.1)

(T₁ : holding temperature after cooling).

The temperature rise (ΔT) in the case of conducting two-steppedtransformation after supercooling is set at a lower limit of 10° C., thetemperature at which softening effect by two-stepped transformationappears, and since the upper limit of the temperature after temperaturerise must not be more than 600° C. the lower limit is set at ΔTdetermined by the following equation:

    ΔT=600-T.sub.1

(T₁ : holding temperature after cooling).

The holding time (T₂) after temperature increase is set as the period upto complete finishing of the transformation.

In the case of mixed two-stepped transformation after temperatureincrease, the supercooling time (t₁) required in the temperature rangeof 350°-500° C. is set at a period after the start of bainitetransformation and of not more than Y sec determined by the followingequation:

    Y=exp (19.83-0.0329×T.sub.1)

(T₁ : holding temperature after cooling).

As in the case of complete two-stepped transformation, the temperaturerise (ΔT) in the case of conducting two-stepped transformation aftersupercooling is set at a lower limit of 10° C., the temperature at whichsoftening effect by two-stepped transformation appears, and since theupper limit of the temperature after temperature rise must not be morethan 600° C. the lower limit is set at ΔT determined by the followingequation:

    ΔT=600-T.sub.1

(T₁ : holding temperature after cooling).

Pearlite texture forms at the wire rod or wire center portion in apearlite wire rod or wire treated at a isothermal transformationtemperature exceeding 500° C. Since pearlite texture has a laminarstructure of cementite and ferrite, it makes a major contribution towork hardening, but a decrease in ductility cannot be prevented. In thehigh area reduction region, therefore, tensile strength increases withan accompanying degradation of twist characteristics, causing theoccurrence of delamination.

In contrast, work hardening is suppressed in the wire rod or wiretransformed in two steps according to this invention since it is in astate of coarse cementite dispersed in ferrite. As a result, it ispossible to suppress occurrence of delamination and enable drawing up tothe high area reduction region.

The bainite texture area ratio is measured from the observed sectionaltexture using the lattice point method. The area ratio is an importantindex indicating the state of bainite texture formation and influencesthe drawability. The lower limit of the area ratio is set at 80%, wherethe two-stepped transformation effect noticeably appears.

The Vickers hardness of the upper bainite structure is an importantfactor indicating the characteristics of the specimen. The cementiteprecipitation in a bainite wire rod or wire which has beentwo-step-transformed by conducting a cooling step and a temperatureincreasing step is coarser than in the case of isothermaltransformation. As a result, the two-step-transformed upper bainitetexture is softened. In consideration of effect on C content the upperlimit of the Vickers hardness is set at not more than 450.

EXAMPLES Example 1

Table 1 shows the chemical compositions of tested steel specimens.

A-D in Table 1 are invention steels and E and F are comparison steels.

Steel E has a C content exceeding the upper limit and steel F has a Mncontent exceeding the upper limit.

The specimens were produced by casting 300×500 mm slabs with acontinuous casting machine and then bloom pressing them into 122-mmsquare slabs.

After these slabs had been rolled into wire rods, they were subjected toDLP (Direct Lead Patenting) cooling under the conditions indicated inTable 2.

The-wire rods were drawn to 1.00 mmφ at an average reduction of area of17% and subjected to tensile test and twist test.

The tensile test was conducted using the No. 2 test piece of JISZ2201and the method described in JISZ2241.

In the twist test, the specimen was cut to a test piece length of 100d+100 and rotated at a rotational speed of 10 rpm between chucks spacedat 100 d. d represents the wire diameter.

The characteristic values obtained in this manner are also shown inTable 2.

No. 1-No. 4 are invention steels.

No. 5-No. 10 are comparative steels.

In comparative steel No. 5, pearlite which formed because the coolingrate was too slow reduced the drawability, leading to breakage duringdrawing.

In comparative steel No. 6, two-step-transformed bainite texture did notform because the temperature rise was too low, reducing the drawabilityand leading to breakage during drawing.

In comparative steel No. 7, martensite formed because a sufficientisothermal transformation period was not secured, reducing thedrawability and leading to breakage during drawing.

In comparative steel No. 8, the ratio of two-step-transformed bainitetexture decreased because the supercooling treatment time was long,reducing the drawability and leading to breakage during drawing.

In comparative steel No. 9, pro-eutectoid cementite which formed becausethe C content was too high reduced the drawability.

In comparative steel No. 10, micromartensite which formed in conjunctionwith central segregation caused by an excessively high Mn contentreduced the drawability.

                  TABLE 1                                                         ______________________________________                                        Chemical Compositions of Tested Steel Specimens                               Chemical Compositions (wt %)                                                  Symbol                                                                              C      Si     Mn   P    S    Cr   Al   Remark                           ______________________________________                                        A     0.95   0.18   0.40 0.006                                                                              0.008                                                                              --   0.002                                                                              Invention                        B     0.98   0.15   0.30 0.006                                                                              0.008                                                                              0.19 0.002                                                                              Invention                        C     1.10   0.16   0.39 0.006                                                                              0.007                                                                              0.21 0.001                                                                              Invention                        D     1.02   0.20   0.35 0.005                                                                              0.008                                                                              0.21 0.002                                                                              Invention                        E     1.30   0.11   0.40 0.005                                                                              0.008                                                                              0.11 0.001                                                                              Comparison                       F     0.98   0.30   1.50 0.006                                                                              0.007                                                                              0.11 0.002                                                                              Comparison                       ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Wire Rod Rolling Conditions and Characteristic Values of Tested Steel         Specimens                                                                                                  Rolled wire rod                                                                             After drawing (diameter: 1.00                                                 mm)                                                  Cooling tank                                                                             TS Reduc-                                                                            Bainite                                                                              TS Reduc-                                                                            Twist                              Diameter                                                                           T.sub.0                                                                          V.sub.1                                                                          T.sub.1                                                                          t.sub.1                                                                            t.sub.2                                                                          kgf/                                                                             tion                                                                              texture                                                                              kfg/                                                                             tion                                                                              value                                                                             Delami-                 No.                                                                              Symbol                                                                            mmφ                                                                            °C.                                                                       °C./s                                                                     °C.                                                                       s ΔT                                                                         s  mm.sup.2                                                                         %   ratio %                                                                           Hv mm.sup.2                                                                         %   (times)                                                                           nation                                                                            Remark              __________________________________________________________________________    1  A   4.0   950                                                                             120                                                                              450                                                                              10                                                                              50  90                                                                              140                                                                              50  95  430                                                                              280                                                                              40  25  No  Invention           2  B   4.5  1000                                                                             150                                                                              450                                                                              15                                                                              50  90                                                                              130                                                                              53  90  420                                                                              300                                                                              42  30  No  Invention           3  C   5.0  1050                                                                             200                                                                              440                                                                              10                                                                              60 110                                                                              140                                                                              58  90  420                                                                              310                                                                              43  28  No  Invention           4  D   5.5   800                                                                             160                                                                              400                                                                               5                                                                              150                                                                              300                                                                              145                                                                              55  85  450                                                                              315                                                                              41  26  No  Invention           5  A   5.5  1000                                                                              50                                                                              450                                                                              20                                                                              100                                                                              150                                                                              150                                                                              25  30  550                                                                              Broke at 1.3                                                                                 Compari-                                                                      son                 6  B   5.0  1050                                                                             130                                                                              450                                                                              20                                                                               0 150                                                                              145                                                                              46  50  480                                                                              Broke at 1.2                                                                                 Compari-                                                                      son                 7  C   5.5  1100                                                                             120                                                                              490                                                                               2                                                                              60 30 140                                                                              15  60  470                                                                              Broke at 1.4                                                                                 Compari-                                                                      son                 8  D   5.5   780                                                                             120                                                                              480                                                                              50                                                                              50 100                                                                              140                                                                              45   0  460                                                                              Broke at 1.3                                                                                 Compari-                                                                      son                 9  E   5.5  1050                                                                             130                                                                              480                                                                              10                                                                              40 100                                                                              170                                                                              35  70  550                                                                              290                                                                              20  13  Yes Compari-                                                                      son                 10 F   5.5  1050                                                                             120                                                                              470                                                                              15                                                                              80 130                                                                              150                                                                              13  60  470                                                                              270                                                                              35  19  Yes Compari-                                                                      son                 __________________________________________________________________________     T.sub.0 : Cooling start temperature                                           V.sub.1 : Cooling rate                                                        T.sub.1 : Holding temperature after cooling                                   t.sub.1 : Holding time after cooling                                          ΔT: Temperature rise                                                    t.sub.2 : Heat treatment time                                            

Example 2

Table 3 shows the chemical compositions of tested steel specimens.

A-D in Table 3 are invention steels and E and F are comparison steels.

The specimens were produced by casting 300×500 mm slabs with acontinuous casting machine, bloom pressing them into 122-mm squareslabs, and producing wire from these slabs.

After heating, these wires were subjected to DLP (Direct Lead Parenting)cooling under the conditions indicated in Table 4.

The wire were drawn to 1.00 mmφ at an average reduction of area of 17%and subjected to tensile test and twist test.

The tensile test was conducted using the No. 2 test piece of JISZ2201and the method described in JISZ2241.

In the twist test, the specimen was cut to a test piece length of 100d+100 and rotated at a rotational speed of 10 rpm between chucks spacedat 100 d. d represents the wire diameter.

The characteristic values obtained in this manner are also shown inTable 4.

No. 1-No. 4 are invention steels.

No. 5-No. 10 are comparative steels.

In comparative steel No. 5, pearlite which formed because the coolingrate was too slow reduced the drawability, leading to breakage duringdrawing.

In comparative steel No. 6, two-step-transformed bainite texture did notform because the temperature rise was too low, reducing the drawabilityand leading to breakage during drawing.

In comparative steel No. 7, martensite formed because a sufficientisothermal transformation period was not secured, reducing thedrawability and leading to breakage during drawing.

In comparative steel No. 8, the ratio of two-step-transformed bainitetexture decreased because the supercooling treatment time was long,reducing the drawability and leading to breakage during drawing.

In comparative steel No. 9, pro-eutectoid cementite which formed becausethe C content was too high reduced the drawability.

In comparative steel No. 10, micromartensite which formed in conjunctionwith central segregation caused by an excessively high Mn contentreduced the drawability.

                  TABLE 3                                                         ______________________________________                                        Chemical Compositions of Tested Steel Specimens                               Chemical Compositions (wt %)                                                  Symbol                                                                              C      Si     Mn   P    S    Cr   Al   Remark                           ______________________________________                                        A     0.95   0.18   0.40 0.006                                                                              0.008                                                                              --   0.002                                                                              Invention                        B     0.98   0.15   0.30 0.006                                                                              0.008                                                                              0.19 0.002                                                                              Invention                        C     1.10   0.16   0.39 0.006                                                                              0.007                                                                              0.21 0.001                                                                              Invention                        D     1.02   0.20   0.35 0.005                                                                              0.008                                                                              0.21 0.002                                                                              Invention                        E     1.30   0.11   0.40 0.005                                                                              0.008                                                                              0.11 0.001                                                                              Comparison                       F     0.98   0.30   1.50 0.006                                                                              0.007                                                                              0.11 0.002                                                                              Comparison                       ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________    Wire Heat Treatment Conditions and Characteristic Values of Tested Steel      Specimens                                                                                                  After heat treatment,                                                         before drawing                                                                              After drawing (diameter: 1.00                                                 mm)                                                  Cooling tank                                                                             TS Reduc-                                                                            Bainite                                                                              TS Reduc-                                                                            Twist                              Diameter                                                                           T.sub.0                                                                          V.sub.1                                                                          T.sub.1                                                                          t.sub.1                                                                            t.sub.2                                                                          kgf/                                                                             tion                                                                              texture                                                                              kfg/                                                                             tion                                                                              value                                                                             Delami-                 No.                                                                              Symbol                                                                            mmφ                                                                            °C.                                                                       °C./s                                                                     °C.                                                                       s ΔT                                                                         s  mm.sup.2                                                                         %   ratio %                                                                           Hv mm.sup.2                                                                         %   (times)                                                                           nation                                                                            Remark              __________________________________________________________________________    1  A   3.0   950                                                                             120                                                                              450                                                                              10                                                                              50  90                                                                              140                                                                              50  95  430                                                                              280                                                                              40  25  No  Invention           2  B   4.0  1000                                                                             150                                                                              450                                                                              15                                                                              50  90                                                                              130                                                                              53  90  420                                                                              300                                                                              42  30  No  Invention           3  C   4.5  1050                                                                             200                                                                              440                                                                              10                                                                              60 110                                                                              140                                                                              58  90  420                                                                              310                                                                              43  28  No  Invention           4  D   5.5   800                                                                             160                                                                              400                                                                               5                                                                              150                                                                              300                                                                              145                                                                              55  85  450                                                                              315                                                                              41  26  No  Invention           5  A   5.0  1000                                                                              50                                                                              450                                                                              20                                                                              100                                                                              150                                                                              150                                                                              25  30  550                                                                              Broke at 1.3                                                                                 Compari-                                                                      son                 6  B   5.0  1050                                                                             130                                                                              450                                                                              20                                                                               0 150                                                                              145                                                                              46  50  480                                                                              Broke at 1.2                                                                                 Compari-                                                                      son                 7  C   4.8  1100                                                                             120                                                                              490                                                                               2                                                                              60  30                                                                              140                                                                              15  60  470                                                                              Broke at 1.4                                                                                 Compari-                                                                      son                 8  D   5.0   780                                                                             120                                                                              480                                                                              50                                                                              50 100                                                                              140                                                                              45   0  460                                                                              Broke at 1.4                                                                                 Compari-                                                                      son                 9  E   4.0  1050                                                                             130                                                                              480                                                                              10                                                                              40 100                                                                              170                                                                              35  70  550                                                                              290                                                                              20  13  Yes Compari-                                                                  son                     10 F   3.5  1050                                                                             120                                                                              470                                                                              15                                                                              80 130                                                                              150                                                                              13  60  470                                                                              270                                                                              35  19  Yes Compari-                                                                  son                     __________________________________________________________________________     T.sub.0 : Wire heating temperature                                            V.sub.1 : Cooling rate                                                        T.sub.1 : Holding temperature after cooling                                   t.sub.1 : Holding time after cooling                                          ΔT: Temperature rise                                                    t.sub.2 : Heat treatment time                                            

Industrial Applicability

As discussed in the foregoing, since the wire rod or wire produced inaccordance with this invention can be drawn to an appreciably higherreduction of area than possible by the prior art method, it has improveddelamination resistance property. The invention enables production ofbainite wire rod and wire excellent in drawability, elimination ofintermediate heat treatment in the secondary processing step, a largereduction in cost, a shortening of production period, and a reduction ofequipment expenses.

We claim:
 1. High-carbon steel wire rod or wire excellent in drawabilitywhich consists essentially of,in weight percent,C: 0.90-1.10%, Si: notmore than 0.40% and Mn: not more than 0.50%, is limited toP: not morethan 0.02%, S: not more than 0.0% and Al: not more than 0.003%, theremainder being Fe and unavoidable impurities, and has a microstructureof, in terms of area ratio, not less than 80% upper bainite textureobtained by two-stepped transformation and an Hv of not more than 450.2. High-carbon steel wire rod or wire excellent in drawability accordingto claim 1 further consisting essentially of Cr: 0.10-0.30% as analloying component.
 3. A method of producing high-carbon steel wire rodexcellent in drawability which comprises,rolling into wire rod a steelslab of a composition which contains, in weight percent,C: 0.90-1.10%,Si: not more than 0.40% and Mn: not more than 0.50%, is limited toP: notmore than 0.02%, S: not more than 0.01% and Al: not more than 0.003%,the remainder being Fe and unavoidable impurities, cooling a rolled wirerod from the temperature range of 1100°-755° C. to a temperature rangeof 350°-500° C. at a cooling rate of 60°-300° C./sec, and holding it inthis temperature range for a specified time period within the range inwhich bainite transformation does not begin or within a range from afterthe start of bainite transformation to prior to completion of bainitetransformation, and increasing the temperature and holding it untilbainite transformation is completely finished.
 4. A method of producinghigh-carbon steel wire rod excellent in drawability according to claim 3wherein the starting slab further contains Cr: 0.10-0.30% as an alloyingcomponent.
 5. A method of producing high-carbon steel wire rod excellentin drawability according to claim 3 which comprises,after the startingslab has been rolled into wire rod, cooling the rolled wire rod from thetemperature range of 1100°-755° C. to the temperature range of 350°-500°C. at a cooling rate of 60°-300° C./sec, holding it in this temperaturerange for not less than 1 sec and not more than a period within therange in which bainite transformation does not begin of X sec determinedby the following equation (1), and increasing the temperature not lessthan 10° C. and not more than 600-T₁ (T₁ : holding temperature aftercooling) °C. and holding it until bainite transformation is completelyfinished,

    X=exp (16.03-0.0307×T.sub.1)                         (1)

where T₁ : holding temperature after cooling.
 6. A method of producinghigh-carbon steel wire rod excellent in drawability according to claim 3which comprisesafter the starting slab has been rolled into wire rod,cooling the rolled wire rod from the temperature range of 1100°-755° C.to the temperature range of 350°-500° C. at a cooling rate of 60°-300°C./sec, holding it in this temperature range for a period from after thestart of bainite transformation to prior to completion of bainitetransformation, specifically for a period of not more than Y secdetermined by the following equation (2), and increasing the temperaturenot less than 10° C. and not more than 600-T₁ (T₁ : holding temperatureafter cooling) °C. and holding it until bainite transformation iscompletely finished,

    Y=exp (19.83-0.0329×T.sub.1)                         (2)

where T₁ : holding temperature after cooling.
 7. A method of producinghigh-carbon steel wire excellent in drawability which comprises,heatingto a temperature range of 1100°-755° C. wire of a composition whichcontains, in weight percent,C: 0.90-1.10%, Si: not more than 0.40% andMn: not more than 0.50%, is limited toP: not more than 0.02%, S: notmore than 0.01% and Al: not more than 0.003%, the remainder being Fe andunavoidable impurities, cooling the heated wire to a temperature rangeof 350°-500° C. at a cooling rate of 60°-300° C./sec, and holding it inthis temperature range for a specified time period within the range inwhich bainite transformation does not begin or within a range from afterthe start of bainite transformation to prior to completion of bainitetransformation, and increasing the temperature and holding it untilbainite transformation is completely finished.
 8. A method of producinghigh-carbon steel wire excellent in drawability according to claim 7wherein the starting wire further contains Cr: 0.10-0.30% as an alloyingcomponent.
 9. A method of producing high-carbon steel wire excellent indrawability according to claim 7 which comprises,cooling the startingwire from the temperature range of 1100°-755° C. to the temperaturerange of 350°-500° C. at a cooling rate of 60°-300° C./sec, holding itin this temperature range for not less than 1 sec and not more than aperiod within the range in which bainite transformation does not beginof X sec determined by the following equation (1), and increasing thetemperature not less than 10° C. and not more than 600-T₁ (T₁ : holdingtemperature after cooling) °C. and holding it until bainitetransformation is completely finished,

    X=exp (16.03-0.0307×T.sub.1)                         (1)

where T₁ : holding temperature after cooling.
 10. A method of producinghigh-carbon steel wire excellent in drawability according to claim 7which comprisescooling the starting wire from the temperature range of1100°-755° C. to the temperature range of 350°-500° C. at a cooling rateof 60°-300° C./sec, holding it in this temperature range for a periodfrom after the start of bainite transformation to prior to completion ofbainite transformation, specifically for a period of not more than Y secdetermined by the following equation (2), and increasing the temperaturenot less than 10° C. and not more than 600-T₁ (T₁ : holding temperatureafter cooling) °C. and holding it until bainite transformation iscompletely finished,

    Y=exp (19.83-0.0329×T.sub.1)                         (2)

where T₁ : holding temperature after cooling.